Lean burn engines, such as diesel and gasoline engines, can provide increased fuel economy and power density. One approach to reduce NOx emissions uses a catalytic converter in the engine exhaust. One type of catalytic converter stores NOx when the engine is running lean, and releases/reduces the stored NOx when a rich exhaust gas enters the catalytic converter. When using a diesel engine, one approach to provide rich exhaust gases to the catalytic converter uses a fuel injector positioned in the engine exhaust.
However, to minimize the amount of excess fuel added to create the rich exhaust gas (due to the large amount of excess oxygen when running lean), a split exhaust gas pipe, each containing a fuel injector and a catalytic converter is utilized. Further, a valve assembly is also used that directs the flow to either branch, and allows reduction of the flow to the branch that is receiving the rich exhaust gas. This requires the downstream injector to overcome only a smaller amount of excess oxygen. Such a system is described in SAE Paper No. 2001-01-3619.
The present inventors, however, have recognized a disadvantage with such an approach. In particular, such a configuration contains two exhaust valves (one for each branch) and two fuel injectors in the exhaust pipes (again, one for each branch). This additional hardware significantly increases system costs, as well as manufacturing complexity and wiring complexity.
The trucking industry is extremely cognizant of increased capital and operating costs of emissions related technology. As described in a recent newspaper article (see “Fleets Wary of New Diesels”, May 31, 2002, The Detroit News, pg. 3B), additional cost of new technology is a major impediment to its implementation.